Preparation

Whisk together flour, baking powder, and salt in a small bowl.

Beat together butter and granulated sugar in a large bowl with an electric mixer at medium-high until pale and fluffy, about 2 minutes in a stand mixer or 4 minutes with a handheld, then beat in sour cream, eggs, and zest. Reduce speed to low and add flour mixture. Mix until just combined. Form into a 2-inch-diameter log.

Set smooth rollers of pasta machine at widest setting. Cut log crosswise into quarters and keep 3 pieces covered with a kitchen towel (not terry cloth). Flatten remaining piece of dough into a rectangle about 1/3 inch thick. Dust with flour. Feed through rollers. Fold in half crosswise and feed through rollers 8 more times, folding in half each time. Dust with flour if sticking.

Turn dial to next (narrower) setting and feed dough through without folding. Continue to feed dough through without folding, making space between rollers narrower each time, until the second to last setting is used.

Cut rolled-out dough into 5- by 2-inch rectangles. Make a 1-inch lengthwise slit in center of each rectangle. Pull one end of strip through slit, forming a loose knot. Transfer to a lightly floured baking sheet.

Heat oil in a 5- to 6-quart heavy pot over moderately high heat until it registers 370°F on thermometer. Fry knots, 4 at a time, turning over once or twice, until golden, about 1 minute per batch. Transfer with a slotted spoon to paper towels to drain.

Turn off heat under oil and roll out and form knots with remaining dough. Return oil to 370°F before frying rest of knots.

Dust cooled cookies with confectioners sugar.

Cooks' notes:
· If you don't have a pasta machine, roll out dough on a well-floured surface with a well-floured rolling pin to less than 1/8 inch thick.
· These cookies are best the day they're made, but they can be kept in an airtight container at room temperature 1 day.